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1.11Ring nematodes

These two ring nematode species (Tylenchida: Criconematidae) have been grouped together because of their similar biology. They are predicted to pose a similar risk and require similar mitigation measures.

The lifecycles and biology of these species are not well documented. The ring nematodes are exclusively root-parasitic, feeding externally on the root surface (Siddiqi 2000). Only females and juveniles are found feeding on plant roots, as the adult males do not have a stylet. Ring nematodes are usually less than 1 mm in size, and are free-living in the soil (Raski and Luc 1987). They show marked sexual dimorphism, with females being cylindrical or sausage-shaped, and males smaller, vermiform and slender (Siddiqi 2000).

Ring nematodes are polyphagous, and may be found among the roots of a number of host plants. Discocriconemella discolabia has been recorded on more than 20 plant hosts (Orton Williams 1980), including yam, pawpaw and sweet potato. Mesocriconema denoudeni has been recorded on more than 65 plant hosts (Orton Williams 1980), including cabbage, capsicum, pawpaw, watermelon, lime, tomato, mango, avocado and sugarcane. It has also been reported as a minor pest of banana in Malaysia, but it is less common than many other nematode species associated with banana (Hassan 2005).

1.11.1Probability of entry

Probability of importation

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  Ring nematodes are ectoparasitic, and feed on the root cortex of host plants, with the anterior of the body thrust into the tissue (Siddiqi 2000).
  
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  Ring nematodes are common in sandy soil and in soils with a high pH (Siddiqi 2000). Nematodes live in the soil around the roots and rhizome of the ginger plant, and so may be present when the ginger is harvested.
  
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  Ring nematodes are migratory ectoparasites that feed on the roots of the host plant (Siddiqi 2000; Raski and Luc 1987). They do not enter the plant tissues, but feed by using their stylet to puncture the plant cells (Luc et al. 1990). Ring nematodes would not typically feed on the rhizome.
  
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  Adult male ring nematodes do not have a stylet and are incapable of tissue feeding (Siddiqi 2000), so will not be physically attached to the ginger. However, the stylet is present in fourth-stage juvenile males (Raski and Luc 1987), and so male juveniles may be present.
  
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  Discocriconemella discolabia is widespread in Fiji, but populations are localised (Orton Williams 1980).
  
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  Discocriconemella discolabia females are small, 0.24–0.30 mm long (Raski and Luc 1987).
  
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  Mesocriconema species nematodes are small, 0.30–0.78 mm long (Siddiqi 2000).
  
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  The small size of these nematodes means nematodes would escape detection during a visual inspection.
  
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  Processing removes the roots from the harvested ginger. The rhizomes are also washed individually with high-pressure water to remove any soil. This would likely remove any ring nematodes present on the surface of the ginger. Given their ectoparasitic feeding behaviour on the roots, these nematodes are unlikely to be present on the rhizomes when harvested. These nematodes would not be present inside the rhizomes.
  

Probability of distribution

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  Dispersal over long distances is most likely to occur with transport of moist soil. As the ginger is washed to remove soil prior to export, the rhizomes should be free of soil.
  
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  Ring nematodes are ectoparasitic root feeders (Siddiqi 2000), and so are unlikely to be present inside the rhizomes.
  
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  In the absence of soil and moisture, ring nematodes would be susceptible to desiccation during distribution.
  
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  Imported ginger may be widely distributed within Australia via retail distribution to supermarkets and greengrocers, and by individual consumers.
  
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  Consumers will carry small quantities of ginger to urban, rural and natural localities. Small amounts of ginger waste could be discarded in these localities.
  
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  Some ginger may be distributed to areas where ginger and other host plant species grow.
  
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  Small amounts of ginger waste could be discarded in domestic compost.
  
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  Living nematodes in discarded ginger waste may be able to find a compatible host in the area where they are discarded, but their ability to move from the rhizome to locate a new host is very limited, and dependant on factors such as soil moisture.
  
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  While ring nematodes would have limited capacity to survive on rhizomes in the absence of soil and moisture, some nematodes could potentially be introduced to the soil if consumers planted rhizomes in backyard gardens. Nematodes would be vulnerable to attack by nematophagous fungi and other microorganisms in the soil. Once roots formed and the ginger established, the nematodes would have a living host on which to feed.
  
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  Active movement of nematodes in the soil is probably limited to several centimetres per year. Movement is dependent on moisture, and will be affected by rainfall, soil texture, compaction and structure, and slope position (Norton and Niblack 1991). Longer distance movement may occur via surface water or wind (Norton and Niblack 1991).
  

Probability of entry (importation × distribution)

1.11.2Probability of establishment

• 
  Ring nematodes feed on the roots of a broad range of plants.
  
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  Mesocriconema denoudeni is polyphagous and has been recorded on more than 65 plant hosts (Orton Williams 1980), many of which are present in Australia, including cabbage, capsicum, pawpaw, watermelon, lime, tomato, mango, avocado and sugarcane.
  
• 
  Discocriconemella discolabia is polyphagous and has been recorded on more than 20 plant hosts (Orton Williams 1980), many of which are present in Australia, including yam, pawpaw and sweet potato.
  
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  There is a low probability that these nematodes would be able to reproduce amphimictically in Australia, because adult males are non-feeding and unlikely to be introduced with imported ginger, provided the ginger is free of soil. The male’s only function is to inseminate the female and then die (Siddiqi 1980). Only inseminated females present a risk.
  
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  However, most Mesocriconema species are parthenogenetic (Luc et al. 1990), and so could establish a population without sexual reproduction.
  
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  Climatic conditions in some parts of Australia will match those of the source areas in Fiji.
  

1.11.3Probability of spread

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  Plant parasitic nematodes require at least a film of water to enable locomotion, and so the soil water content is a primary ecological factor (Luc et al. 1990).
  
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  These nematodes are most likely to be spread through the movement of infested soil, particularly on farm equipment and plant material.
  
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  Ring nematodes move sluggishly, crawling like a worm by the lengthening and shortening of the body annules (Siddiqi 2000). Dispersal over long distances is most likely to occur with transport of moist soil.
  
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  If these nematodes established in growing areas, it is possible that they could remain undetected for some time, initially causing little noticeable damage, and may be inadvertently spread via planting stock.
  
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  Spread is also possible by transfer to alternative hosts and propagation via that pathway.
  
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  Natural spread would be slow, relying on dispersal by water. Nematodes only actively move several centimetres per year (Norton and Niblack 1991).
  

1.11.4Probability of entry, establishment and spread

1.11.5Consequences

1.11.6Unrestricted risk estimate

Unrestricted risk is the result of combining the probability of entry, establishment and spread with the outcome of overall consequences. Probabilities and consequences are combined using the risk estimation matrix shown in Table 2.5.

The unrestricted risk estimate for ring nematodes of ‘negligible’ achieves Australia’s ALOP. Therefore, specific risk management measures are not required for this pest.